Optical alignment method and arrangement for practicing the method

ABSTRACT

A method and arrangement for optical alignment of spatially separated objects in which a laser beam of substantially coherent radiation has disposed therein a phase plate producing a minimum of intensity, a diffraction diaphragm representing the object to be aligned and an observation screen on which appear the characteristics of the diffraction pattern produced by the diaphragm. Photoelectric scanning elements may be arranged in the plane of said observation screen for detecting said diffraction pattern.

[ 72] Inventor ll-lans Dieter Betz Mannheim, Germany [21] Appl. No.728,208

[22] Filed May 10,1968

[45] Patented Nov. 2, 1971 [73] Assignee Carl Zeiss-Stiftung Heidenheimon the Brenz, Wuerttemherg, Germany [32] Priority Feb. 20, 1968 [33]Germany [54] OPTICAL ALIGNMENT METHOD AND ARRANGEMENT FOR PRACTICHNG THEMETHOD 4 Claims, 7 Drawing Figs.

[52] US. Cl 356/172, 350/ l 62 [51] Int. Cl ..G0lbl1/26 e, v. 356/172,1l0,1ll;350/162,163

[56] References Cited UNITED STATES PATENTS 3,240,108 3/1966 Lehan etal.356/71 Primary Examiner-Ronald L. Wibert Assistant ExaminerT. MajorAtt0rney -Singer, Stern & Carlberg ABSTRACT: A method and arrangementfor optical alignment of spatially separated objects in which a laserbeam of substantially coherent radiation has disposed therein a phaseplate producing a minimum of intensity, a diffraction diaphragmrepresenting the object to be aligned and an observation screen on whichappear the characteristics of the diffraction pattern produced by thediaphragm. Photoelectric scanning elements may be arranged in the planeof said observation screen for detecting said diffraction pattern.

PATENTED 2 3,617, 1 35 SHEET 10F 2 3a 3b 4 5 H92 g/ac r Fig.3

OPTICAL ALIGNMENT METHOD AND ARRANGEMENT FOR PRACTICING THE METHOD Theinvention relates to a method and an arrangement for optical alignmentof spatially separated objects.

With the customary adjusting methods which employ a boresight, it isonly possible to obtain, at the most, accuracies of about I rad.

Another conventional method which employs tensioned wires can only beused for distances which at the most do not exceed a few hundred meters.

It is also known to employ a light ray path through the zero plane ofthe light which is produced by the interference of two oppositely phasedlight bundles, for instance with the assistance of one dimensional"phase plates (see H. Wolter, Annalen der Physik 6, 1950, pgs. 341-368).

It is an object of the invention to create an optical alignment methodwhich pennits a definite determination of target displacements of about0.0I mm. at distances up to a few kilometers (l0 rad).

According to the invention this is made possible by employing a laserbeam diffracted with the assistance of a phase element and by disposingthe objects to be aligned at the resulting minimum intensity of thislaser beam.

The fine adjustment of the targets then takes place preferably byobservation of the symmetry condition of a diaphragm diffractionpattern. The detection of the diffraction pattern may take placevisually or also with the assistance of photoelectric scanning devices.

The advantages of the method of the invention reside not only in thesimplicity and the accuracy obtained, but also in the fact that thegeometry of a corresponding arrangement may be selected in afar-reaching variability and that an alignment may be obtained for shortdistances and also for distances which extend over a number ofkilometers with almost the same accuracy.

With these objects in view the invention will hereinafter be describedin greater detail with reference to the accompanying drawings, in which:

FIG. 11 illustrates diagrammatically and in a side elevation view theprinciple construction of an arrangement for practicing the method ofthe invention;

FIG. 2 illustrates an elevation view of a so-called onedimensional phaseplate;

FIGS. 3, d, and 5 illustrate intensity characteristics;

FIG. 6 illustrates diagrammatically a two-dimensional alignmentarrangement, and

FIG. 7 presents the way to use an additional telescope with or without aspatial filter. A spatial filter can also be utilized at the end of theconsidered system.

Referring to FIG. 1, a laser beam source I emits a narrow bundle of rays2 of a substantially coherent radiation of constant amplitude. Thisbundle of rays with the assistance of a phase plate of conventionalconstruction and which, for instance, comprises a transparent carrier 3ahaving arranged thereon aA/2 layer 3b, is split in two coherent parts ofopposite phase.

FIG. 2 illustrates in an elevation view such a so-called onedimensional"phase plate which is employed during an alignment of targets in onedimension extending about perpendicular to the plane of the drawing. Theboundary line of the M2 layer 3b is designated with 30 (range of thephase transition). In FIG. I such a phase plate, by way of example, isvertically inserted into the path of the bundle of rays in such a mannerthat the boundary line 30 extends perpendicular to the plane of thedrawing so that the bundle of rays 2 is substantially split into halves.

In any optional distance in rear of the phase plate is produced in viewof Fresnels diffraction phenomenon an intensity distribution of theradiation which in accordance with the boundary line 3c of the )t/2layer 3b has a central absolute minimum of intensity zero.

This can easily be proven by an interception screen 4 which is insertedinto the path of the bundle of rays, see the dash line 4' in FIG. 1. InFIG. 3 an example of the characteristic of the intensity is indicatedwith 5.

In the arrangement illustrated in FIG. 1 is also shown the arrangementof a diaphragm 6 which symbolizes the target to be aligned. Thisdiaphragm 6 is provided with a slit having the width d (the length ofthe slit extends perpendicular to the plane of the drawing) and isarranged at the point of the minimum intensity zero of the radiation.The diffraction pattern produced on this diaphragm 6 is observed on theprojection screen 7. If now the center line 8 of the slit in thediaphragm 6 has a small distance A from the course of the minimum linezero, then one observeson the screen 7 an intensity characteristic 9 asshown by way of example in FIG. 4. Thereby the position of the centralminimum zero corresponds to the position of the line zero in FIG. I. Thetwo maxima of the intensity which are adjacent to the central minimumzero differ from each other for example by an amount A].

If one now displaces during the observation of the diffraction patternon the screen 7 the diaphragm 6 in the direction of the arrow 10 until asymmetrical diffraction pattern is obtained, substantially as it appearsin FIG. 5 and is indicated with 9' (A.I=0), then the center line 8 ofthe slit in the diaphragm 6 and the minimum line zero are brought intocoincidence with each other and this means that the target representedby the slit has been aligned with the reference plane zero.

The evaluation of the intensity conditions which as described relies ona visual observation may also be accomplished with the assistance ofphotoelectric receivers 11 (FIG. 1) which are relatively slidable withrespect to the laser beam path and may comprise well-known multipart"photo elements.

It is believed to be obvious that an alignment of targets as illustratedin FIG. I and which takes place in one predetermined dimension may alsobe performed by rotating all parts about an angle of so that thealignment takes place in a dimension at right angles to the firstmentioned dimension.

The principle of the present invention permits, however, to make use ofthe advantage which arises when both these features are employedsimultaneously in one arrangement.

Such a two-dimensional alignment arrangement is illustrated by way ofexample in a diagrammatic perspective view in FIG. 6. In this FIG. 6 thelaser beam source is designated with The two-dimensional" phase plate 3'has two square zones 3'b covered each with M2 layers which arerelatively offset to each other in such a manner that two boundary lines3:: and 3y are produced which are perpendicular to each other in a planeextending at right angles to the center axis of the laser beam path. Itis on these boundary lines where the phase transitions of occur. If onenow places in the minimum intensity (designated by the axis Z) of theFresnel diffraction pattern a diaphragm 6' having a square aperture 6",then one is ableto observe on the projection screen 7' a reticle X, Ywhich corresponds to the path of the two intersecting axes of theintensity zero of the diffraction pattern, as well as four maximumintensities I, II, III, and IV.

If the center of the aperture of the diaphragm 6'-which diaphragm 6'represents the target to be aligned-is correctly aligned upon the axis2, then the diffraction pattern appearing on the screen 7' is completelysymmetrical and the four maxima of intensity I, II, III, and IV have allthe same size. Otherwise similar asymmetry effects occur as have beendescribed in connection with the embodiment according to FIG. I.

The asymmetry effect during the adjustment of the diaphragm is a directmeasure of the deviation from the reference lines upon which theadjustment is made.

For illustrating the size of this asymmetric effect it may be stated,for example, that in an arrangement according to FIG. 1 in which thewidth of the slit 6 is d=l mm. and in which the displacement of thisslit is A=0.0l mm. in an optional distance from the source of the rays 1there is obtained an asymmetric intensity All] of about 10 percent.

Since it depends solely concerning the asymmetric effect to be observedupon a relative displacement of the diffraction slit and the minimumintensity of the structural laser beam, it is obvious that thediffraction slit and the observation screen may also be mounted fixedlyand that one or more phase plates-which now represent the targets to bealigned-are inserted into the laser beam without causing the describedmethod to be changed in any manner.

The last-named arrangement has now the additional advantage that aplurality of objects may be aligned with a single laser arrangement.

The divergence of the laser beam in a manner known may be reduced by theinsertion of a telescope l2 focused to infinity into the laser beam andin an inverted position, i.e. with its ocular facing the source of thelaser beam as illustrated in F IG. 7.

In the arrangements in accordance with the invention, the telescope 12is preferably inserted between the source of the laser beam 1" and thephase plate 30'', 3b", 30" since otherwise the minimum of intensitywhich is being produced is also enlarged (FIG. 7). it is of advantage toinsert a spatial filter 13 into the telescope 12 as shown in FIG. 7 inorder to mask nonparallel distorting rays. A corresponding spatialfilter 14 can be used close to the plane of detection as indicated inFIG. 7 in order to purify the minimum and to improve the alignmentaccuracy.

Obviously, the method of the present invention is not limited to thealignment of targets as described in the above embodiments, but may beemployed with advantage wherever any objects in an arrangement have tobe aligned accurately with respect to one another, for instance duringthe adjustment of focusing elements of a linear accelerator, during theadjustment of target marks on geodetical measurements, during theguidance of excavation machines or other moving objects, during thecoupling of flying objects (space vehicles), etc. In addition, theobservation of fixed objects should be mentioned, in order to detecteventual small movements.

What I claim is:

l. A method of optically aligning a laser source, a phase plate and anobject containing an optical diaphragm, comprising the steps ofproducing a primary diffraction pattern containing a central absoluteintensity minimum by projecting a laser beam through the phase plate andsplitting said laser beam into fractional beams in phase opposition witheach other, projecting said diffraction pattern toward a detectionplane, producing a secondary diffraction pattern having four principalsymmetrically arrayed intensity maxima at said detection plane byinserting said diaphragm in alignment between the phase plate and thedetection plane, and moving said phase plate and diaphragm relative toeach other trans versely of said laser beam to equalize the intensity ofsaid four maxima, whereby alignment is achieved.

2. An apparatus for optically aligning a laser source, a phase plate andan object containing an optical diaphragm, comprising in the followingorder: a laser source emitting substantially coherent radiation, a phaseplate in the path of the laser beam for splitting said laser beam intofractional beams in phase opposition and thereby producing a primarydiffraction pattern containing a central absolute intensity minimum,said optical diaphragm in the laser beam producing a secondarydiffraction pattern having four intensity maxima, means for causingrelative displacement between said diaphragm and phase platetransversely of the laser beam, and detector means for detectingdifferences in intensity between said four intensity max- 3. Anapparatus according to claim 2, including a telescope disposed in saidlaser beam between said laser source and said phase plate for reducingthe divergence of said laser beam.

4. An apparatus according to claim 3, including a spatial filter in saidtelescope and another spatial filter close to the detector means forpurifying the beam and improving the alignment accuracy.

1. A method of optically aligning a laser source, a phase plate and anobject containing an optical diaphragm, comprising the steps ofproducing a primary diffraction pattern containing a central absoluteintensity minimum by projecting a laser beam through the phase plate andsplitting said laser beam into fractional beams in phase opposition witheach other, projecting said diffraction pattern toward a detectionplane, producing a secondary diffraction pattern having fOur principalsymmetrically arrayed intensity maxima at said detection plane byinserting said diaphragm in alignment between the phase plate and thedetection plane, and moving said phase plate and diaphragm relative toeach other transversely of said laser beam to equalize the intensity ofsaid four maxima, whereby alignment is achieved.
 2. An apparatus foroptically aligning a laser source, a phase plate and an objectcontaining an optical diaphragm, comprising in the following order: alaser source emitting substantially coherent radiation, a phase plate inthe path of the laser beam for splitting said laser beam into fractionalbeams in phase opposition and thereby producing a primary diffractionpattern containing a central absolute intensity minimum, said opticaldiaphragm in the laser beam producing a secondary diffraction patternhaving four intensity maxima, means for causing relative displacementbetween said diaphragm and phase plate transversely of the laser beam,and detector means for detecting differences in intensity between saidfour intensity maxima.
 3. An apparatus according to claim 2, including atelescope disposed in said laser beam between said laser source and saidphase plate for reducing the divergence of said laser beam.
 4. Anapparatus according to claim 3, including a spatial filter in saidtelescope and another spatial filter close to the detector means forpurifying the beam and improving the alignment accuracy.